The Presocratic period begins in 585 BCE but has no fixed endpoint. It lasts until Socrates changes the focus of the discussion. Just when this happened is not clear, but it must have happened well before he was executed by the city of Athens in 399 BCE. Not all Presocratics died before Socrates. Democritus seems to have lived until about 370 BCE.
It is not easy to know what the Presocratics thought, in part because what they wrote has survived only in fragments and in reports from later in time. Aristotle (384-322 BCE) thinks of them as "ancient" (Metaphysics I.986b), and he and his student Theophrastus (late fourth to early third century BCE) are the primary source for much of what we know about the Presocratics.
(The great majority of ancient texts that have survived through transmission are those of Plato, Aristotle, and their commentators most of whom were Neoplatonists. These texts account for over ninety-plus percent of what has survived. This was not the result of chance. The dominant schools had an interest in certain texts, and these were the ones that were transmitted.)
The texts for understanding the Presocratics are not as a whole (as far as I know) freely available in translation on the internet. (The Hanover Historical Texts Collection has some of them). A now standard source book is Early Greek Philosophy, edited and translated by André Laks, Glenn W. Most. This is in the Loeb Classical Library and is available through ASU.
The Three Parts of the Presocratic Period
The Presocratic Period divides into three main parts. It begins in the 6th century BCE in the city of Miletus (which is on the coast of Asia Minor (modern day Turkey)) with Thales and his fellow Milesian naturalists. Parmenides challenges this new beginning. The Atomists, Democritus and Leucippus, try to bring together the insights of these two prior traditions.
Thales (Θαλῆς, Thalēs), active between the second half of the 7th century and the first decades of the 6th century BCE
Anaximander (Ἀναξίμανδρος, Anaximandros), active in the middle of the 6th century BCE
Anaximenes (Ἀναξιμένης, Anaximenēs), active in the middle to the early part of the 6th century BCE
Thales, Anaximander, and the Anaximenes are Inquirers into Nature.
Parmenides (Παρμενίδης, Parmenidēs), active in the early to the middle part of the 5th century BCE
Democritus (Δημόκριτος, Dēmokritos), c. 460 - c. 360 BCE
Leucippus (Λεύκιππος, Leukippos), 5th century BCE
Democritus and Leucippus are the ancient Atomists.
Some Historical Events in the World of Ancient Greece
The Mycenaeans were the dominate culture until about the 12th century BCE. Mycenae is a city in in the northeastern Peloponnesus. (The name of the city derives from Homer. Agamemnon has his palace and home at Mycenae.) The political organization was in terms of a palace economy. The monarch and his family controlled everything. They held a royal domain that contained most of the wealth of the city. They appointed bureaucratic officials to manage this domain and the economy, which consisted primarily in collectivized agriculture and trade.
The Collapse of the Palatial Centres
In the 12th century BCE, this palace-centered civilization completely collapses. The result is the Dark Ages, in which writing and trade ceased with the collapse of the palatial centres and corresponding economy in terms of which society was organized. Out of this collapse, a new beginning took place cut off from the great civilizations of Mesopotamia and Egypt.
This resulted in city-states. These city-states were republics. They were not like the cities of the prior Mycenaean Civilization. The differences in wealth among the citizens was small. There were no monarchial rulers who controlled everything. There was no bureaucracy because there was no royal holdings that needed management. There was no caste of priests who controlled religious practice. There were no mercenary soldiers. There was no money to hire them. The citizens themselves had to defend their cities. As independent defenders of their cities, they demanded a role in political decision-making. In this way, the life of the citizen in the "city-state" (πόλις) is political. (The Greek word πόλις transliterates as polis, and it is the etymological root of the English word 'politics.')
People fleeing the collapse of the Mycenaean world moved east to Athens, further east to the islands of the Aegean Sea, and even further to the west coast of Asia Minor. Greeks speaking the Ionic dialect settled in Asia Minor on the coast of the Aegean Sea. This part of the coast came to be called "Ionia," and it was here, particularly in Miletus, the dominant city, that the enlightenment took root.
Contact with the Great Civilizations
Beginning in about 8th century BCE, there was a marked increase in trade and colonization throughout the eastern Mediterranean. This brought increasing awareness of the surrounding civilizations of Mesopotamia, Egypt, and, later, Persia. The Greeks, however, were not overwhelmed by these cultures. They assimilated them in a way that would give rise to a philosophical tradition.
Greek and Persian contact came in the form of the Persian Wars, a series of military conflicts between the Achaemenid Empire of Persia and Greek city-states.
By about 550 BCE, the Persian Empire had expanded westward. Greek cities on the eastern shore of the Aegean, including Miletus, came under Persian rule. In 492 BCE, the Persians invaded in the northern part of the Greek peninsula. In 490 BCE, against all odds, the Greeks were victorious against the Persians at Marathon. In 480 BCE, the Persians launched a second invasion. Again they were defeated again. This time at sea in the Straits of Salamis.
The Fall of Athens in the Peloponnesian War
In the aftermath of the Persian wars, the Athenians, who played a leading role in winning the war, set up a league of city-states (the Delian League) to clear the Aegean of Persian power. At the same time, the Athenians used funds from the league to rebuild and transform Athens in a way that made it the center of the Greek world. This led to conflict among the members of the league and eventually to the Peloponnesian War in 431 BCE. The war would end with Athen's defeat in 404 BCE.
The City of Athens executes Socrates in 399 BCE
In part because he was blamed for Athen's demise, Socrates was executed in 399 BCE. Sometime between his birth in about 470 BCE and his execution in 399 BCE, he changed the focus of philosophical discussion from the inquiry into nature to matters involving ethics and the good life. This change of focus traditionally marks the end of the Presocratic Period.